With thousands of radio dishes spread over thousands of kilometres, the Square Kilometre Array will be the largest and most powerful radio telescope in the world and able to explore the deepest regions of the cosmos. But only if these thousands of radio dishes, some hundreds of kilometres apart, can be synchronized to within a fraction of a billionth of a second.
When it comes to telescopes, bigger is better. The larger they are, the more light they can catch, making them more sensitive to the faint light from stars and galaxies in the distant universe, and the better their resolution. But there are limits to how big you can build a telescope. The largest single-dish radio telescope in the world is China’s FAST telescope at 500 metres across. To go bigger, and make a more powerful telescope, engineers need to use a technique called aperture synthesis. In aperture synthesis, the signals from two or more smaller telescopes are fed into a computer that combines them into a single image, simulating a single telescope equivalent in size to the distance between the individual smaller telescopes. Using aperture synthesis, telescope engineers can place radio dishes kilometres or even thousands of kilometres apart, creating a telescope with thousands of times greater resolution than FAST.
Aperture synthesis is not new. The technique was first used by Australian radio astronomers in 1946 using a decommissioned radar antenna to study the Sun. Since then, telescopes like the Very Large Array in New Mexico, which has 27 dishes spread up to 36 km apart, have used the technique to transform our understanding of the universe. Very-Long-Baseline Interferometry, with dishes thousands of kilometres apart, takes aperture synthesis to the extreme, and gives us our most detailed look at galaxies in the furthest reaches of deep space.
The Square Kilometre Array (SKA) is a next generation radio telescope due to begin construction in the coming months. The SKA will have hundreds of radio dishes spread over hundreds of kilometres. (A planned upgrade will eventually expand the telescope to thousands of dishes spread over thousands of kilometres.) This vast scale introduces many new engineering challenges.
One of those challenges is how to synchronize all of the dishes. For aperture synthesis to work properly, the astronomical signals received by each dish need to be synchronized to less than 0.005 nanoseconds by the tick of an atomic clock at the heart of the array. Transmitting this atomic clock tick to the dishes is difficult. Over long distances, disturbances on the transmission cables such as vibration or temperature changes degrade the atomic clock signal. After about 30 km, the signal has become too degraded to synchronize the dishes to the precision required, which would lead to blurrier images. Smaller arrays like the Very Large Array and the Australia Telescope Compact Array can get away without having to worry about this because their dishes are less than 30 km from the atomic clock. Very-Long-Baseline Interferometry gets around this problem by having a separate atomic clock at each dish. But with hundreds, and eventually thousands of dishes spread over vast distances, the SKA would need dozens of atomic clocks, a solution that would be prohibitively expensive.
We developed a system that is able compensate for the disturbances affecting the atomic clock signal, allowing the tick to be transmitted much longer distances, and at much lower cost and greater scalability than previous similar systems. Our central transmitter takes the atomic clock tick, encodes it onto a laser, and sends it to each of the hundreds of dishes through fibre-optic cables. (In the same way your internet data is transmitted.) A small mirror in the receiver at each dish reflects a portion of the laser signal back to the central transmitter. The electronics in the transmitter compare the returning signal (which has picked up twice the disturbance because of its two trips through the fibre link) to the signal that was transmitted, works out in what way the signal has been degraded, and modifies the outgoing transmission to compensate for the disturbances. Our system is able to maintain the precision of the atomic clock signal to better than 5 parts in a trillion at dishes up to 175 km from the atomic clock. If your watch were that good, it would only lose one second every 600 thousand years.
We had previously published details of the invention and testing of this technology, as well as details of its performance during trials on a working radio telescope, but the purpose of this paper is to provide a public record of how the system will actually be installed and operated on the SKA, as well as provide new details about its performance during temperature, seismic resilience, and electromagnetic interference compliance tests.
The SKA is actually split into two telescopes: SKA-MID which will be built in South Africa, and SKA-LOW which will be built in Western Australia. Our system will synchronize the hundreds of dishes of SKA-MID, while a system developed by colleagues at Tsinghua University in China will synchronize the thousands of smaller antennas that make up SKA-LOW.
The SKA, while mighty, will not be the last big telescope. Future giant telescopes, like the Next Generation Very Large Array, will also need signal stabilization systems like ours, but with even better performance. We are already looking to develop a system to meet the much more challenging requirements of optical astronomy, which might pave the way for optical telescopes on the scale of the SKA or Very Large Array. Beyond astronomy, systems like ours, able to transmit atomic clock time signals with greater precision than current standards, will have uses in synchronizing telecommunications networks, provide the extreme timing accuracy required in commerce such as high-frequency trading, and make GPS navigation much more precise. It’s only a matter of time.